Typically, a multi-site trunked communication system includes communication units, communication sites, communication resources, and a communication resource allocator, also called a central controller. Each site has a substantially distinct coverage area and is geographically located throughout the system. A number of communication resources are assigned to each site, where at least one of the communication resources is used as a control channel, while the remaining communication resources are used as voice and/or data channels, hereafter referred to simply as channels. Base stations or repeaters are used to generate the communication resources and are located at the site. The communication resources may be telephone connections, TDM time slots, carrier frequencies, and/or frequency pairs.
The communication units, also called subscribers, are arranged in talk groups and may be located anywhere within the system (in the coverage area of any site). When a subscriber from a talk group requests a group call, it transmits, via the control channel of the site in which it is located, an inbound signalling word (ISW) to the central controller. A group call allows all members of the same talk group that are located within the system to communicate with each other. The ISW generally comprises the requesting subscriber's individual identification number, the requesting subscriber's talk group, and a request for a group call. The ISW may contain more or less information, depending on the system.
Upon receiving the ISW, the central controller determines if there is an available channel in each site. If a channel is not available in each site, the central controller will not place the group call and instead transmits an outbound signalling word (OSW) to the requesting subscriber. The OSW generally comprises, at least, a site busy signal, which indicates that the group call was not placed because at least one site did not have an available channel. When a channel becomes available in each site, and the requesting subscriber has not cancelled the group call request, the central controller allocates a channel in each site to the requesting subscriber's talk group.
At times, when a mobile subscriber unit is active in a group or individual call, the mobile unit travels out of the coverage area of its current site, causing the call to become degraded and even lost. The call may then be handed off to another site for completion. One method of call hand-off begins when a subscriber, while receiving a call, determines that it must switch to a new site to complete the call because the subscriber is at the edge of the RF range of its current site. The subscriber finds the next best control channel and issues an ISW at the new site. The central controller acknowledges the request to continue the call. If a channel is available at the new site, the central controller assigns a channel and issues a channel grant for the radio. The radio accepts the channel grant and goes to the assigned channel to continue the call. If no channel is available, the central controller issues a busy OSW for this call and the subscriber is unable to continue the call at the new site.
The subscriber requesting registration at a new site for call completion is likely to be issued a busy OSW if the system has a relatively high volume of calls. He may lose the call completely before a new channel is assigned. Since the user has his subscriber to receive important calls, this is not acceptable.